31 PREGNANCY RATE OF CROSSBRED COWS SUBMITTED TO FIXED-TIME AI AND ADMINISTRATION OF GONADOTROPIN-RELEASING HORMONE ANALOGUE TWELVE DAYS AFTER AI

Abstract

The importance of serum progesterone (P4) during the first weeks of pregnancy to reduce the embryo mortality in cattle herds has been demonstrated (Mann and Lamming 2001 Reproduction 121, 175-180). Studies show that a P4 peak at the beginning of pregnancy (about 5 days after the natural breeding or AI) helps development of the concept via secretion of interferon-τ (Mann 2002 XXII World Buiatrics Congress 18-23, 300-306). Studies have shown that the administration of gonadotropin-releasing hormone (GnRH) or its agonists after AI may stimulate corpus luteum function, induce the formation of an accessory corpus luteum, or increase of P4 serum concentrations and, therefore, improve embryonic survival (Bartolome et al. 2005 Theriogenology 63, 1026-1037). This work aims to evaluate the effect of GnRH injection 12 d after fixed-time AI(FTAI) on P4 serum levels and pregnancy rate of beef crossbred cows. The cows (Bos taurus indicus × Bos taurus taurus, n = 59), range in body condition (BCS) from 2.5 to 3.5 (scale of 1 to 5), were not previously pregnant. The animals were allocated to 2 treatments: TEB treatment (n = 30) consisted of Day 0 insertion of progesterone intravaginal device (ID) plus 2 mg of estradiol benzoate (EB), i.m. followed on Day 8 by removal of the ID and 300 IU of eCG and 0.15 mg of PGF_2α, i.m. on Day 9, 1 mg injection of EB (i.m.) was given, followed by FTAI at 48 to 56 h after the withdrawal of ID. The TEBGnRH12 treatment (n = 29) protocol was similar to TEB, but with administration of 25 μg of GnRH analogue (Lecirelina) 12 d after the FTAI. Transrectal ultrasound was used for pregnancy diagnosis on Day 35 after AI. Blood samples for P4 serum assay were collected from 30% of animals in each treatment, at the time of AI and on Days 5, 12, and 20 after FTAI. For all statistical analysis, we used the program SAS 9.0 (2002; SAS Institute Inc., Cary, NC, USA) at 5% probability. The pregnancy rate was analyzed by logistic regression. The effects of treatment and day in P4 concentration were analyzed in a split plot, with the effect of day in plot, in mixed model, considering the error and repetition as a random effect, and comparison of means by Tukey-Kramer. Pregnancy rate and the concentration of serum P4 on different days was analyzed by Spearman correlation. The pregnancy rate for TEB cows was 53.33% (16/30) for the first service compared with 37.93% (11/29) for TEBGnRH12 cows. The protocol used did not affect the pregnancy rate of crossbred cows (P > 0.05). There were no treatment differences (P > 0.05) between the concentrations of P4 (TEB = 3.88 ng mL^-1; TEBGnRH12 = 3.12 ng mL^-1). In conclusion, the administration of the analogue of GnRH 12 days after FTAI does not affect the rates of pregnancy or the concentration of P4.

Supported by grants from CNPq, FAPEMIG and DZO-UFV.